A trick of stereo viewing whereby, if a single vertical line is presented to one eye and two vertical lines to the other, and one of the double lines is fused with the single line in binocular viewing, the unmatched line is perceived to be nearer or further away than the fused line. A concept used in the design of stereo mounting grids. A phenomenon first described by the scientist Panum in 1858.

parallax

Generally, the differences in a scene when viewed from different points (as, photographically, between the viewfinder and the taking lens of a camera).

In stereo, often used to describe the small relative displacements between homologues, more correctly termed deviation.

parallax stereogram

A form of autostereogram which currently describes a technique in which alternate thin vertical strips of the left and right hand views are printed in a composite form and then overlaid with a grating (originally), or (nowadays) a lenticular sheet of cylindrical lenses which presents each view to the correct eye for viewing stereoscopically.

photogrammetry

A professional discipline which uses stereography as a basis for scientific measurement and map-making.

planar (image)

Flat, or in a single plane; as opposed to stereoscopic or three-dimensional.

polarisation (of light)

The division of beams of light into separate planes or vectors by means of polarising filters (first practically applied by Edwin Land of the Polaroid company in the 1930s). When two vectors are crossed at right angles, vision or light rays are obscured. By polarising the light from two projectors in opposite vectors, and by providing viewers with spectacles having similarly polarising filters, each image from a stereo pair - when projected onto a type of screen surface which does not then depolarise the light rays - is seen only by the left and right eye respectively.

The amount or method by which film is advanced between exposures in a purpose-built stereo camera. The Colardeau progression moves by an even two frames; the Verascope progression moves by one and three frames alternately.

pseudoscopic, or pseudo (coll.)

The presentation of three-dimensional images in inverse order, so that the farthest object is seen as closest and vice-versa: more correctly referred to as inversion. Achieved (either accidentally or deliberately, for effect) when the left and right images are transposed for viewing.

Pulfrich effect

Term now used to describe an illusory stereoscopic effect which is produced when two-dimensional images moving laterally on a single plane (as on a film or television screen) are viewed at slightly different time intervals by each eye, the perceived delay between the eyes being achieved by means of reduced vision in one of them; eg, through the use of a neutral-density filter. The apparent positional displacement which results from this is interpreted by the brain as a change in the distance of the fused image. A scene is produced giving a depth effect, the depth being proportionate to the rate of movement of the object, not to the object distance. The phenomenon was first adequately described in 1922 by Carl Pulfrich, a physicist employed by Carl Zeiss, Jena, in relation to a moving object (a laterally-swinging pendulum).